1. Field of the Invention
The present invention generally relates to an optical amplifier and in particular to an optical amplifier using an optical fiber amplifier pumped by a light source.
2. Description of the Related Art
The noise characteristic of an optical fiber amplifier is one of the important performance items. To reduce in noise level, there has been disclosed an optical amplifier having two rare-earth-doped fibers connected in series through an optical isolator (see Japanese Patent Unexamined Publication No. 7-30178). The optical isolator can block ASE (Amplified Spontaneous Emission) propagating from the second rare-earth-doped fiber at the output stage, resulting in reduced noise at the first stage.
According to the conventional optical circuit of an optical fiber amplifier, however, ASE generated in the first rare-earth-doped fiber at the input state passes through the optical isolator into the second rare-earth-doped fiber. Therefore, the second rare-earth-doped fiber amplifies the ASE as well as the signal light, resulting in reduced efficiency and output power. In other words, the pumping energy of the light pumping the second rare-earth-doped fiber is consumed to amplify not only the signal light but also the ASE.